WR has got this all wrong. DOF for a given camera postion (keeping the subject/background ratio the same) and a given f-stop VARIES with focal length used because the in-camera magnification (ratio of object sizes to lens image sizes ) changes if you change the focal length and keep he camera in the same postion. The DOF **DOES CHANGES** as well as the AOV as you change focal lengths. Any basic phootograpy student would know this from experience alone, let alone a textbook or webpage explanation. Try shooting a focused subject at five feet with a detailed background at 10 feet. Use a 135mm lens at F2.8 and a 20mm lens at F2.8. You will easily see that the background is MUCH more in focus (obviously different DOF) in the 20mm shot because the magnification is way less EVEN THOUGH the camera postion/ ratio of foreground to background and f-stop are the same. jco
-----Original Message----- From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] On Behalf Of William Robb Sent: Saturday, April 28, 2007 12:08 PM To: Pentax-Discuss Mail List Subject: Re: M85mm f2.0 bokeh ----- Original Message ----- From: "Tom C" Subject: RE: M85mm f2.0 bokeh > Certainly what you state regarding DOF is true. I believe though the > writer > was not making a dogmatic absolute statement of scientific fact. He was > generalizing. > >>As the camera: subject/background ratio wasn't altered, DOF should be >>SIMILAR for all four lenses. > > I'm pretty sure he knows that actual DOF is not changed by altering > the subjects distance from the focal plane. That's only moving > subjects in to, out of, or within the range referred to as DOF. I > think he means that the > *perceived* DOF will be *similar*, which is true for lenses close to > the same focal length used at close to the same aperture. Not the > same, but similar. Depth of field is not subject to changes in focal length if the ratio of camera to subject to background is not altered, presuming the aperture is a constant. What happens if you change focal length is that the angle of view changes, nothing else. If you take a picture with a 35mm lens , then change lenses to a 100mm lens, and take a picture using the same camera position and aperture, you will note that the depth of field is identical (presuming a precise aperture match), but you need to magnify the picture taken with the 35mm lens rather substantially to match the picture taken with the 100mm lens. Wikipedia has a rather extensive article regarding this subject at: http://en.wikipedia.org/wiki/Depth_of_field William Robb -- PDML Pentax-Discuss Mail List [email protected] http://pdml.net/mailman/listinfo/pdml_pdml.net -- PDML Pentax-Discuss Mail List [email protected] http://pdml.net/mailman/listinfo/pdml_pdml.net
